The present disclosure relates generally to hard drive disks, and more particularly, to short yoke length coils for magnetic heads in disk drives.
Data is stored on magnetic media using a write head. Writing involves storing a data bit by utilizing magnetic flux from a write head to set the magnetic moment of a particular area on the magnetic media, typically a magnetic disk. An actuator arm moves the magnetic head on an arc across the rotating disk, thereby allowing the magnetic head to access the entire disk. The state of the magnetic moment is later read, using a read head, to retrieve the stored information. Data density is determined by the amount of data stored on an area of magnetic media and depends on how much area must be allocated to each bit. Data on a magnetic disk is stored in multiple concentric tracks, where increasing the number of bits per track and tracks per disk increases data density. Typically, the bits are orientated perpendicular to the disk. These systems, known as PMR systems, reduce the size of the segment required to represent one bit of information through the perpendicular orientation of the magnetization, thereby increasing the areal density.
Data density, or areal density, is determined by both the bit length and by the width of the bit. Areal density is equal to the bits-per-inch along the track times the tracks-per-inch radially on the disk. To decrease bit size, head size is decreased by fabricating thin film read and write heads.
A thin film write head consists of three principal functional parts, a yoke (having two poles), a pancake shaped copper coil wound around the yoke, and a front gap between the poles, all made from thin films. The coil, which is sandwiched by the two poles (a write pole and a return pole), converts electrical signals (a write current) into magnetic fields. The yoke poles, which are conventionally separated by a gap layer, coil, and insulation layers, is used to form a low-reluctance (high permanence) magnetic circuit and to deliver the magnetic flux generated by the coil to the front gap. Insulation layers between the coil and the poles are typically made of hard-cured photoresist. The stray field from an air bearing surface (ABS) in close proximity to the magnetic disk is used for writing.
The write current applied to the coil is in general rectangular wave shape pulses. When rectangular wave shape pulses are applied, the coil inductance affects the temporal current induced magnetic field response, which depends upon the structure of the thin-film magnetic head, upon an output impedance of a current source connected with the coil, and upon a frequency and a voltage of the applied rectangular wave pulses. These are affected also by a characteristic impedance of trace conductors and connection lines between the current source and the magnetic head. Particularly, when the influence of the trace conductor is eliminated by fixing the frequency and the current of the applied pulses, this variation in the wave shape of the current is caused by non-linearity of the input impedance of the coil.
If the wave shape of the current flowing through the inductive write head element of the thin-film magnetic head is deformed, magnetic patterns written in a magnetic medium will become distorted and thus write and read operations of data will become difficult. Also, in order to improve the non-linear transition shift (NLTS) in dynamic characteristics, it is necessary to shorten a rising time of the wave shape of the current flowing through the coil.
Therefore, the requirements for the wave shape of the current flowing through the coil are to maintain a profile of the rectangular wave shape pulses provided from the current source as much as possible, to have a short rising time, and to have a high current value while holding the rectangular wave shape in order to obtain a strong write magnetic field.
These requirements may be satisfied by decreasing the coil inductance at the frequency of the write current. However, if the number of turns of the coil is reduced to decrease the inductance, magnetic force generated from the coil will decrease causing no improvement of the characteristics. Also, if the size of the coil is reduced by narrowing a coil pitch, difficulty in fabrication of the coil may occur with conventional fabrication procedures.
For high data rate writer application, one of the requirements is to have fast saturation and low inductance to induce short rise time. Using photoresist for insulation may result in limiting minimum dimensions in reducing the yoke length and coil. A shorter coil yoke length may provide an advantage with respect to data rate characteristics of the write head.